Wave filter



W. P. MASON WAVE FILTER Filed July 27, 1935 Fl .l'

Max 1 r 4 0.2 0.3 0,4 0,5 RATIO OF OPTICAL TO MECHANICAL AXIS lNVE/VTORMR MASON i A TTDRNE) Patented May 25, 1937 PATENT OFFICE WAVE FILTERWarren P. Mason, West Orange, N. J., assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication July 2'7, 1935, Serial No. 33,511

13 Claims.

This invention relates to wave filters of the type employingpiezoelectric crystals as reactance elements.

An object of the invention is to extend the frequency range of such wavefilters.

Another object is to reduce the cost of filters of this type.

A further object is to reduce the number of piezoelectric elementsrequired in the construction of such crystal filters.

Still further objects of the invention are to support a piezoelectricelement for unrestricted vibration and to simplify the making ofelectrical contact with a plurality of electrodes associated with eachmajor face of the element.

A feature of the invention is a wave filter employing as a reactanceelement a piezoelectric crystal adapted to vibrate in the flexural mode.

Another feature is the use in such a filter of a piezoelectric crystalcut in the form of a tuni fork.

,Another feature is a piezoelectric crystal adapted to vibrate infiexure and serving to take the place of two crystal elements ordinarilyrequired.

Still another feature of the invention is a. piezoelectric crystalhaving a plurality of electrodes associated with each major facesupported at a nodal region by a plurality of pair of clamps which areutilized to make electrical contact with the electrodes.

Heretofore piezoelectric crystals have been used as reactance elementsin the construction of wave filters but difficulty has been encounteredin obtaining crystals which will vibrate at a sufiiciently low frequencythat the transmission bands may be located at comparatively lowfrequencies. In accordance with the present invention piezoelectriccrystals adapted to vibrate in the flexural mode are employed in theconstruction of such filters and the frequency range is therebyconsiderably extended. Crystal elements out either in the form of a baror in the form of a tuning fork may be used for this purpose, and theelements are preferably supported at or near their nodal points. As anextension of the invention a single crystal element is made to take theplace of two elements ordinarily required in the construction of such afilter. In one form of the invention the supports which hold the crystalelement are utilized for the purpose of making electrical connections tothe electrodes of the crystal.

The nature of the invention will be more fully understood from thefollowing description and by reference to the accompanying drawing, ofwhich:

Fig. 1 shows a piezoelectric crystal element cut in the form of a barand adapted to vibrate in the flexural mode;

Fig. 2 is an end view of the crystal element of Fig. 1 showing how itmay be supported by clamps which are used to make electrical contactwith the individual electrodes;

Fig. 3 shows a piezoelectric crystal element cut in the form of a tuningfork, with its associated electrodes and clamping members;

Fig. 4 is a sectional view of the crystal element of Fig. 3 taken alongthe line 4, 4;

Fig. 5 shows how the electrodes of the elements shown in Figs, 1 and 3may be connected so that the crystal will vibrate in the flexural mode;

Fig. 6 shows how four of the elements of Figs. 1 or 3 may be arranged inthe form of a latticetype wave filter;

Fig. '7 represents a graph of data useful in the design of such filters;and

Fig. 8 is a schematic representation of a lattice-type wave filter inwhich two piezoelectric elements take the place of four such elements.

One form of the piezoelectric crystal element used in the invention isshown in Fig. 1 which is a parallelopiped it having a pair of electrodes[2 and I3 associated with one major face and a second pair of electrodesl4 and 15 associated with the opposite face. When such a bar is set intovibration in the flexural mode as explained hereinafter it will vibrateabout two nodal lines represented by l6 and I1, located approximately0.224 of the length of the bar from its ends. In order that the leastdamping of the vibrations will be introduced by the holder, it ispreferable to support the bar at or near these points. This may be done,for example, by means of the clamps 28 and M which engage the electrodesl2 and [3, respectively, on one side of the crystal, and an oppositelydisposed pair of clamps 22 and 23 which engage the electrodes I4 and i5on the other side of the crystal near the line H. A third pair of clamps24 and 25, and a fourth oppositely disposed pair of clamps engage thecrystal electrodes in the region of the nodal line it.

As shown in Fig. 2, the clamps described above may, for example, bemetal inserts inlaid into the supporting members 26 and 2'! which aremade of insulating material. Electrical connectors may be soldered orotherwise secured to the clamps as indicated at the points 28, 29, 30and 3!, for the purpose of interconnecting the various electrodes or forconnecting the crystal element into an external circuit.

Fig. 3 shows a second form of the crystal ele ment, cut in the form of atuning fork having two prongs 32 and 33, and a butt part 34. On one sideof the tuning fork is an electrode 35 extending along the outer edges ofthe prongs. and along the lower portion of the butt. A secand electrode36 extends along the inner edges fork. Such a tuning fork will have anodal region which follows a perpendicular line, such as the one shownat 39, bisecting the butt part. The fork is preferably supported alongthis nodal line, which may be done, for example, by means of a pair ofclamps 4| and 42 on one side, and an oppositely disposed pair of clamps43 and 44 on the other side. As shown in Fig. 4 these clamps may bemetal inserts set into a pair of supporting members made of insulatingmaterial, and electrical connections to the electrodes may be made bysoldering connectors to these clamps, as explained above in connectionwith Fig. 2.

Fig. 5 is a schematic diagram showing how the electrodes of the crystalelement of Fig. 1 may be connected together in order to set up aflexural vibration in the bar. The electrode 12 and the diagonallyopposite electrode l5 are connected to one terminal 45 and the tworemaining electrodes are connected to the other terminal 46. Theseconnections may be made by means of the connectors and conductingclamps, as explained above. When an alternating electronictive force isimpressed upon terminals 45 and 46 the element II will be set intovibration in the flexural mode. Piezoelectric elements of this typeadapted to vibrate fiexurally are disclosed in greater detail in U. S.Patent 1,823,320 issued September 15, 1931 to W. A. Marrison to whichreference is hereby made.

The frequency of vibration in the flexural mode for a zero degree, X-cutcrystal having a mechanical axis one centimeter in length is shown incurve 41 of Fig. 7, which gives the frequency in kilocycles per secondplotted against the ratio of optical to mechanical axis. By a a zerodegree, X-cut crystal is meant one cut from a. mother crystal having aprincipal face which is perpendicular to a face of the mother crystaland having a width dimension which makes a zero angle with the opticalaxis. Curve 48 of the figure presents the same data for a -18 degree,X-cut crystal, that is, one having a width dimension which makes anangle of -18 degrees with the optical axis. The thickness of theelectrical axis plays no part in the determination of the frequency. Fora crystal of any other length the frequency can be determined from theprincipal of similitude which states that for a crystal of a given shapethe resonant frequency of any mode is inversely proportional to themagnitude of any dimension. For a zero degree crystal five centimeterslong, for example, with the ratio of optical to mechanical axis of 0.2it will be seen from curve 41 that the frequency is about 20 kilocycles.This is only about one-third of the frequency for the same crystal whenvibrating in the longitudinal mode. For a 18 degree cut crystal thefrequency will be somewhat less, as shown by curve 48, due to the factthat Youngs modulus is less for this cut.

A well known representation of the equivalent electrical circuit of apiezoelectric crystal is a capacitance C1 shunted by an arm comprising asecond capacitance C2 in series with an inductance. The value of theratio for a crystal in which the electrodes on one side cover fromtwo-thirds to four-fifths of the surface is about 180 for the l8 degreecut crystal and about for the zero degree cut crystal. The shuntcapacitance Cl. of the equivalent network will be the electrostaticcapacitance between the two sets of plates. From this data the values ofthe reactances in the equivalent circuit may be determined for a barvibrating in ilexure.

Fig. 6 shows how two pairs of crystal elements adapted to vibrate inflexure may be arranged to form a lattice-type wave filter. The networkhas a pair of input terminals 41, 48 and a pair of output terminals 49,50, with one pair of the elements 5|, 52 connected in the seriesbranches and the other pair 53, 54 connected diagonally be tween the twosets of terminals. The electrodes of the crystal are connected asindicated in Fig. 5.

With crystal elements cut in the form of a bar and vibrating in theflexural mode, frequencies as low as 16 or 17 kilocycles may beobtained. In accordance with the invention still lower frequencies maybe obtained by the use of a piezo- 3 electric crystal element out in theform of a tuning fork as shown in Fig. 3. In order to cause the tuningfork to vibrate the electrodes are connected as shown in Fig. 5, in thiscase the electrodes l2, l3, l4 and I5 representing the electrodes 35,35, 3'! and 38 of the tuning fork crys' tal. The two outside electrodes35 and 3'! for a certain voltage polarity will cause the outside of thetwo prongs to expand, while at the same time the voltage applied to thetwo inside electrodes 36 and 38 will cause the inside parts of the twoprongs to contract, thus forcing the prongs inward and making themvibrate in the form of a tuning fork.

The frequency f of a tuning fork in cycles per second is given by theformula which is included in the prong, having an individual prong widthof 0.4 centimeter will vibrate at the frequency With a crystal 6centimeters in length and of reasonable width it is possible, therefore,to get down to a frequency of the order of 1 kilocycle. This type ofelement, with a reasonable size, can also be made to work as high as 16kilocycles. Thus, by the use of piezoelectric elements in the form of abar supplemented by those in the form of a tuning fork, it is possibleto cover the frequency range extending from 1 to 50 kilocycles withoutusing a crystal larger than is required for vibrations in thelongitudinal mode at 50 ki1ocycles,

The ratio C1 to C2 for the zero degree cut crystal in the form of atuning fork is about 300. By means of the above data the reactances inthe equivalent circuit for the tuning fork crystal may be evaluated. Twopairs of such tuning fork crystals may be arranged in the form of alatticetype filter, the schematic diagram being the same as that shownin Fig. 6.

I =1284 cycles (9.)

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ti l

Fig. 8 shows how in accordance with an extension of the invention twopiezoelectric crystal elements are made to take the place of four suchelements in the construction of a wave filter. As shown in the figure,two crystal elements 55 and 56, either of the type shown in Fig. 1 orthe type shown in Fig. 3, are arranged between a pair of input terminals51, 58 and a pair of output terminals 59, 60 to form a lattice-typenetwork. One set of oppositely disposed electrodes asso ciated withcrystal 55 are connected between input terminal 5'! and thecorresponding output terminal 59, while the other set of electrodes isconnected between the other two terminals 58 and 6D. This single elementthus effectively furnishes the two impedance branches connected inseries with the line. The diagonal impedance branches are furnished bythe other crystal element 56, one set of electrodes being connectedbetween terminals 58 and 58 while the other set of electrodes isconnected between terminals 51 and 6 3. When two elements are made totake the place of four as shown in Fig. 8 the transmissioncharacteristics of the resulting filter will be the same as whenindividual elements are used in each impedance branch, but thecharacteristic impedance of the network will be doubled.

What is claimed is:

1. A wave filter comprising as a reactance element a piezoelectriccrystal in the form of a tuning fork having two prongs connected by abutt part, each of two opposite faces of said crystal having anelectrode extending along the outer edges of said prongs and along saidbutt, and a second electrode of opposite polarity extending along theinner edges of said prongs, and said electrodes being connected into thefilter circuit in such a way that the applied electrical stresses causethe outside parts of said prongs alternately to expand and contract andthe inner parts of said prongs alternately to contract and expand.

2. A wave filter having a plurality of pairs of impedance branchesconnected between a pair of input terminals and a pair of outputterminals to form a lattice network, said filter comprising apiezoelectric crystal adapted to vibrate in the fiexural mode, and saidcrystal taking the place of a plurality of separate crystal elements.

3. In a wave filter comprising a plurality of impedance branches equalin pairs connected between two input terminals and two output terminals,a piezoelectric crystal adapted to vibrate in the flexural mode, saidcrystal providing reactances which are efiective in a plurality of saidbranches and taking the place of twoseparate crystal elements.

4. In a wave filter of the lattice type comprising a plurality ofimpedance branches equal in pairs, two piezoelectric crystals adapted tovibrate in the flexural mode, each of said crystals pr viding reactanceswhich are efiective in a plurality of said branches, and said twocrystals serving to take the place of four separate crystal elements.

5. In a four-terminal transmission network comprising a plurality ofbranch impedances, two of said impedances being adapted to determine thetransmission characteristics of said ne work, a piezoelectric crystal inthe form of a tuning fork, said crystal providing reactances which areeffective in a plurality of said branch impedances and taking the placeof a plurality of separate crystal elements.

6. In a four-terminal transmission network comprising a plurality ofbranch impedances, two of said impedances being adapted to determine thetransmission characteristics of said network, a piezoelectric crystaladapted to vibrate in the flexural mode, said crystal providingreactances which are effective in a plurality of said impedances andtaking the place of a plurality of separate crystal elements.

'7. In a four-terminal transmission network comprising two pairs ofequal impedances arranged to form a symmetrical lattice, a piezoelectriocrystal in the form of a tuning fork, said crystal providing reactanceswhich are effective in each of the branches forming one of said pairs.

8. In a four terminal transmisison network comprising two pairs of equalimpedance branches arranged to form a symmetrical lattice, apiezoelectric crystal adapted to vibrate in the flexural mode, saidcrystal providing reactances which are effective in each of the branchesforming one of said pairs.

9. A wave filter comprising two pairs of equal impedance branchesconnected between two input terminals and two output terminals to form asymmetrical lattice network, said filter comprising as a reactanceelement a piezoelectric crystal having a pair of electrodes placedadjacent corresponding portions of opposite sides of said crystalparallel to its longest axis and unsymmetrically with respect to saidaxis, a second pair of electrodes associated with said opposite sidesand interconnections between said electrodes, whereby said crystal tendsto vibrate in. the fiexural mode to provide reactances which areeffective in each of the branches forming one of said pairs.

10. A piezoelectric crystal element in the form of a tuning fork, twoelectrodes associated with one side of said element, two otherelectrodes associated with the opposite side of said element, and twopairs of conducting clamps for supporting said element along a nodalline and for making electrical contact with each of said electrodes.

11. In a wave filter of the lattice type comprising a pair of equalseries impedance branches and a pair of equal diagonal impedancebranches, two piezoelectric crystals adapted to vibrate in the flexuralmode, one of said crystals providing reactances which are effective ineach of said series branches, and the other of said crystals providingreactances which are effective in each of said diagonal branches.

12. In a wave filter of the lattice type comprising a pair of equalseries impedance branches and a pair of equal diagonal impedancebranches, two piezo electric crystal elements in the form of tuningforks, one of said elements providing reactances which are effective ineach of said series branches, and the other of said elements providingreactances which are effective in each of said diagonal branches.

13. A wave filter comprising as a reactance a piezoelectric crystalelement in the form of a tuning fork, said element having two electrodesassociated with one face, two other electrodes associated with theopposite face, two pairs of conducting clamps for supporting saidelement along a nodal line and for making electrical contact with eachor said electrodes, and connectors for interconnecting said electrodes.

WARREN P. MASON.

